A suspension is an important element of a motor vehicle and significantly influences the traveling stability and riding comfort of the motor vehicle. The suspension mainly consists of a link (arm, rod and the like) for supporting a tire to a vehicle body in a vertically moveable fashion, a spring for absorbing an impact from a road surface by deformation, and a damper for damping the vertical oscillation of the vehicle body. As a damper in such a suspension system, a so-called cylinder-type damper is widely used. A cylinder-type damper mainly consists of a cylinder tube filled with a hydraulic oil and a piston rod having a piston at its end, wherein the piston is received in the cylinder in a slidably moveable fashion and the hydraulic oil is adapted to move among a plurality of oil chambers in response to the movement of the piston.
Recently, in an effort to improve the performance of the cylinder-type damper, a variable damping force damper for variably controlling the damping force depending on a dynamic state of a motor vehicle has been developed. The variable damping force damper was mainly embodied as a mechanical-type damper in which the piston is provided with a rotary valve for varying an orifice area and the rotary valve is rotatably driven by an actuator. However, in order to simplify the structure and improve the operation response, it has been proposed to use a Magneto-Rheological fluid (MRF) as the hydraulic oil and control the viscosity of the MRF by using a magnetic fluid valve provided to the piston (see Japanese Patent Application Laid-Open Publication No. 2006-77789, for example). In the variable damping force damper of this type, an electric current supplied to the magnetic fluid valve is detected by a current detection circuit and a feedback control is conducted based on the result of the current detection.
In the variable damping force damper using MRF, a pulse width modulation (PWM) switch circuit is typically used to control the damping force by increasing or decreasing the electric current supplied to the magnetic fluid valve. As a result, electromagnetic noises having frequencies of multiples of the PWM signal frequency (e.g., 20 kHz) are generated from the wirings for conducting electric current supplied from the control device to the variable damping force damper, and the electromagnetic noises can interfere with radio signal reception or the like. For such reasons, the inventors conceived providing an LC filter consisting of an inductor and capacitor at a downstream of the PWM switch circuit to reduce the electromagnetic noises higher than a prescribed frequency.
However, a resonance at a certain frequency cannot be avoided in the LC filter, and as a result, the current detection circuit may be applied with a signal beyond its detection range and this can lead to inaccurate current detection or decrease in durability of the current detection circuit. Further, in the case where the resonance of the LC filter occurs at a frequency near the control frequency of the variable damping force damper, the control can be disturbed by the resonance and this can unfavorably affect the behavior of the motor vehicle.